Chapter 4

Tissues/Histolgy

Tissues

• Groups of cells similar in structure and function

• The four types of tissues

• Epithelial

• Connective

• Muscle

• Nerve

Epithelial Tissue

• Cellularity – composed almost entirely of cells

• Special contacts – form continuous sheets held together by tight junctions and desmosomes

• Polarity – apical and basal surfaces

• Supported by connective tissue – reticular and basal laminae

• Avascular but innervated – contains no blood vessels but supplied by nerve fibers

• Regenerative – rapidly replaces lost cells by cell division

Classification of Epithelia

• Simple or stratified

• Squamous, cuboidal, or columnar

Epithelia: Simple Squamous

• Single layer of flattened cells with disc-shaped nuclei and sparse cytoplasm

• Functions

• Diffusion and filtration

• Provide a slick, friction-reducing lining in lymphatic and cardiovascular systems

• Present in the kidney glomeruli, lining of heart, blood vessels, lymphatic vessels, and serosae

Epithelia: Simple Cuboidal

• Single layer of cubelike cells with large, spherical central nuclei

• Function in secretion and absorption

• Present in kidney tubules, ducts and secretory portions of small glands, and ovary surface

Epithelia: Simple Columnar

• Single layer of tall cells with oval nuclei; many contain cilia

• Goblet cells are often found in this layer

• Function in absorption and secretion

• Nonciliated type line digestive tract and gallbladder

• Ciliated type line small bronchi, uterine tubes, and some regions of the uterus

Epithelia: Pseudostratified Columnar

• Single layer of cells with different heights; some do not reach the free surface

• Nuclei are seen at different layers

• Function in secretion and propulsion of mucus

• Present in the male sperm-carrying ducts (nonciliated) and trachea (ciliated)

Epithelia: Stratified Squamous

• Thick membrane composed of several layers of cells

• Function in protection of underlying areas subjected to abrasion

• Forms the external part of the skin’s epidermis (keratinized cells), and linings of the esophagus, mouth, and vagina (nonkeratinized cells).

Epithelia: Stratified Columnar

• Several cell layers with cuboidal basal cells and columnar superficial cells

• Functions in protection and secretion

• Present in large ducts of some glands, and in portions of the male urethra

Epithelia: Transitional

• Several cell layers, basal cells are cuboidal, surface cells are
dome shaped

• Stretches to permit
the distension of the urinary bladder

• Lines the urinary bladder, ureters, and part of the urethra

Glandular Epithelia

• A gland is one or more cells that makes and secretes an aqueous fluid

• Classified by:

• Site of product release – endocrine or exocrine

• Relative number of cells forming the gland – unicellular or multicellular

Endocrine Glands

• Ductless glands that produce hormones

• Secretions include amino acids, proteins, glycoproteins, and steroids

Exocrine Glands

• More numerous than endocrine glands

• Secrete their products onto body surfaces (skin) or into body cavities

• Examples include mucous, sweat, oil, and salivary glands

• The only important unicellular gland is the goblet cell

• Multicellular exocrine glands are composed of a duct and secretory unit

Multicellular Exocrine Glands

• Classified according to:

• Simple or compound duct type

• Structure of their secretory units

Modes of Secretion

• Merocrine – products are secreted by exocytosis (e.g., pancreas, sweat, and salivary glands)

• Holocrine – products are secreted by the rupture of gland cells (e.g., sebaceous glands)

Connective Tissue

• Found throughout the body; most abundant and widely distributed in primary tissues

• Connective tissue proper

• Cartilage

• Bone

• Blood

Functions of Connective Tissue

• Binding and support

• Protection

• Insulation

• Transportation

Characteristics of Connective Tissue

• Connective tissues have:

• Mesenchyme as their common tissue of origin

• Varying degrees of vascularity

• Nonliving extracellular matrix, consisting of ground substance and fibers

Structural Elements of Connective Tissue

• Ground substance – unstructured material that fills the space between cells

• Fibers – collagen, elastic, or reticular

• Cells – fibroblasts, chondroblasts, osteoblasts, and hematopoietic stem cells

Ground Substance

• Interstitial (tissue) fluid

• Adhesion proteins – fibronectin and laminin

• Proteoglycans – glycosaminoglycans (GAGs)

• Functions as a molecular sieve through which nutrients diffuse between blood capillaries and cells

Fibers

• Collagen – tough; provides high tensile strength

• Elastic – long, thin fibers that allow for stretch

• Reticular – branched collagenous fibers that form delicate networks

Connective Tissue: Fundamental Cell Type

• Fibroblasts – connective tissue proper

• Chondroblasts – cartilage

• Osteoblasts – bone

• Hematopoietic stem cells – blood

• White blood cells (WBCs), plasma cells, macrophages, and mast cells

Connective Tissue (CT): Embryonic

• Mesenchyme – embryonic CT

• Gel-like ground substance with fibers and star-shaped mesenchymal cells

• Gives rise to all other connective tissues

• Found in the embryo

Connective Tissue Proper: Loose

• Areolar CT

• Gel-like matrix with all three CT fibers

• Fibroblasts, macrophages, mast cells, and some WBCs

• Wraps and cushions organs

• Widely distributed throughout the body

Connective Tissue Proper: Loose

• Adipose CT

• Matrix similar to areolar CT with closely packed adipocytes

• Reserve food stores, insulates against heat loss, and supports and protects

• Found under skin, around kidneys, within abdomen,
and in breasts

Connective Tissue Proper: Loose

• Reticular CT

• Loose ground substance with reticular fibers

• Reticular cells lie in a fiber network

• Forms a soft internal skeleton, or stroma, that supports other cell types

• Found in lymph nodes, bone marrow, and the spleen

Connective Tissue Proper: Dense Regular

• Parallel collagen fibers with a few elastic fibers

• Major cell type is fibroblasts

• Attaches muscles to bone or to other muscles, and bone to bone

• Found in tendons, ligaments, and aponeuroses

Connective Tissue Proper: Dense Irregular

• Irregularly arranged collagen fibers with some elastic fibers

• Major cell type is fibroblasts

• Withstands tension in many directions providing structural strength

• Found in the dermis, submucosa of the digestive tract, and fibrous organ capsules

Connective Tissue: Cartilage

• Hyaline cartilage

• Amorphous, firm matrix with imperceptible network of collagen fibers

• Chondrocytes lie in lacunae

• Supports, reinforces, cushions, and resists compression

• Forms the costal cartilage

• Found in: embryonic skeleton, the end of long bones, nose, trachea, and larynx

• Elastic cartilage

• Similar to hyaline cartilage but with more elastic fibers

• Maintains shape and structure while allowing flexibility

• Supports external ear (pinna) and the epiglottis

• Fibrocartilage

• Matrix similar to hyaline cartilage but less firm with thick collagen fibers

• Provides tensile strength and absorbs compression shock

• Found in intervertebral discs, the pubic symphysis, and in discs of the knee joint

Connective Tissue: Bone (Osseous Tissue)

• Osseous tissue

• Hard, calcified matrix with collagen fibers found in bone

• Osteocytes are found in lacunae and are well vascularized

• Supports, protects, and provides levers for muscular action

• Stores calcium, minerals, and fat

• Marrow inside bones is the site of hematopoiesis

Connective Tissue: Blood

• Blood

• Red and white cells in a fluid matrix (plasma)

• Contained within blood vessels

• Functions in the transport of respiratory gases, nutrients, and wastes

Epithelial Membranes

• Cutaneous – skin

• Mucous – lines body cavities open to the exterior
(e.g., digestive and respiratory tracts)

• Serous – moist membranes found in closed ventral body cavity

Nervous Tissue

• Branched neurons with long cellular processes and support cells

• Transmits electrical signals from sensory receptors to effectors

• Found in the brain, spinal cord, and peripheral nerves

Muscle Tissue: Skeletal

• Long, cylindrical, multinucleate cells with obvious striations

• Initiates and controls voluntary movement

• Found in skeletal muscles that attach to bones or skin

Muscle Tissue: Cardiac

• Branching, striated, uninucleate cells interdigitating at intercalated discs

• Propels blood into the circulation

• Found in the walls of the heart

Muscle Tissue: Smooth

• Sheets of spindle-shaped cells with central nuclei that have no striations

• Propels substances along internal passageways (i.e., peristalsis)

• Found in the walls of hollow organs

Tissue Trauma

• Causes inflammation, characterized by:

• Dilation of blood vessels

• Increase in vessel permeability

• Redness, heat, swelling, and pain

Tissue Repair

• Organization and restored blood supply

• The blood clot is replaced with granulation tissue

• Regeneration and fibrosis

• Surface epithelium regenerates and the scab detaches

• Fibrous tissue matures and begins to resemble the adjacent tissue

• Results in a fully regenerated epithelium with underlying scar tissue

Developmental Aspects

• Primary germ layers: ectoderm, mesoderm, and endoderm

• Three layers of cells formed early in embryonic development

• Specialize to form the four primary tissues

• Nerve tissue arises from ectoderm

• Muscle, connective tissue, endothelium, and mesothelium arise from mesoderm

• Most mucosae arise from endoderm

• Epithelial tissues arise from all three germ layers